Prosthetic materials are engineered elements which can achieve biological function when placed within a living organism. An important class of prosthetic materials are those which are used to repair and replace human body tissue such as osseous matter. To replace biological tissue in an acceptable, long lasting manner, the replacement materials must join with the surrounding living matter. Proper melding is achieved through the use of an appropriate material having a micro-network of capillaries permeating the structure to permit living tissue in-growth.
Such porous networks must be continuous, permitting unrestricted passage of blood and other body fluids from the surrounding tissue while also providing structural support. This can be easily envisioned in the design of artificial bone wherein osseous replacement materials must support the forces and stresses associated with the skeletal system and simultaneously allow passage of blood gases, nutrients, waste products and other extracellular material to and from the surrounding tissue.
In reconstructive surgery such as repair of highly comminuted fractures, healing can be accelerated by inclusion of materials having such porous matrix adjacent the break point to enhance bone growth. Rebuilding of damaged long bones can also benefit from insertion of such porous prosthetic materials to re-achieve the desired pre-damage shape and strength.
Such porous yet semi-rigid materials are found in nature. For example, spiny starfish, certain sea urchins and coral exhibit a solid structure formed of calcium carbonate having a network of interconnecting pores and significant void volume in the form of a micro-porous matrix. Specifically, the slate pencil sea urchin has cigar-shaped protrusions that have a void volume of 50 percent and a porous structure with pore diameters of approximately 25 .mu.m. Certain coral provide similar attributes with pore diameters of approximately 250-600 .mu.m.
In the past, these aquatic materials were used to form biologically acceptable structures such as through hydrothermal treatment of the calcium carbonate skeletons to form hydroxyapatite. More detailed discussion of such techniques may be found in U.S. Pat. Nos. 3,890,107, 3,929,971, 4,231,979, 4,722,870 and 4,861,733, the teachings of which are incorporated by reference herein.
Although these procedures offer a unique class of structures, they are accompanied by several significant drawbacks. The naturally forming aquatic structures are never completely uniform and often exhibit imperfections detrimental to surgical implantation. In addition, the materials are expensive to harvest, and such gleaning of nature has raised environmental impact concerns in some quarters.
These problems have led to a search for techniques to engineer and manufacture porous materials having specifically delineated structural properties in a controlled manner. In this search, applicant has developed a unique collection of porous articles of the type discussed above. These are disclosed in U.S. patent application Ser. No. 07/647,999 (filed Jan. 30, 1991) (U.S. Pat. No. 5,348,788) identified above, the contents of which is incorporated by reference herein as if restated in full.
Related thereto, applicant has developed several important advancements which are described herein below.